For a formation to contain hydrocarbons and permit the hydrocarbons to flow through it, the rock comprising the formation must have certain well known physical characteristics. One characteristic is that the formation has a certain measurable resistivity (the inverse of conductivity), which can be determined by appropriate transducers in the drill string. Analysis of the data from these transducers provide information regarding the resistivity of the formation surrounding the resistivity tool, which then can be used in combination with other measurements to predict whether the formation will produce hydrocarbons. In addition, a sudden measured change in resistivity at the boundary between various strata can be used to locate these boundaries.
Because the drilling mud in the borehole is commonly maintained at a higher pressure than the formation, the mud tends to invade the permeable formation surrounding the borehole, forcing the original connate water to be driven away from the borehole. This flushing of drilling mud filtrate into the formation creates an invaded or flushed zone around the borehole, with a transition zone between the flushed and undisturbed zones. This invasion may effect accurate measurements of formation resistivity. The depth of invasion of the drilling mud is a factor of the formation porosity, the differential drilling pressure, permeability of the formation, water loss of the drilling fluid, and time. In order to reduce the invasion effects, it is desirable to measure the resistivity as close to the bit as possible. To detect strata boundaries, it is desirable to detect formation resistivity ahead of the bit.